Tank Car Design and Use Characteristics 41 EXPLANATION OF TANK CAR DESIGN SPECIFICATION NUMBERS (General American Transportation Corporation 1991) DOT 111 A 60 AL W : : : Welded construction : : Construction metal other than steel (alumi- : : num) : Tank test pressure Insignificant unless S, T, or J (see below) Class number Authorizing agency Authorizing Agency Tank cars that meet DOT or AAR specifications are designated with the appropriate specification number beginning with either prefix. AAR cars meet most DOT requirements and are authorized to carry some regulated commodities. Class Number Class numbers denote general categories of tank cars that have several common design and construction features. The most common tank car is the DOT-111, which accounts for about three-quarters of the tank car fleet. The 111, which is a nonpressure design, carries most kinds of materials moved by tank car. The next most common classes are the DOT-105 and DOT-112 pressure tank cars, which account for between 20 and 25 percent of tank cars in service and a higher share of tank cars used in hazardous materials service. "A," "S," "J," and "T" Identifiers The letters A, S, J, or T follow the class number of some tank car designs. Whereas A has no special significance except as a separator of the class and test pressure numbers, the other letters indicate that the car is equipped with certain protective systems. S indicates that the car is equipped with head protection. J indicates that the car is equipped with head protection and jacketed thermal protection. T indicates that the car is equipped with head protection and sprayed-on thermal protection. These designations apply mainly to the pressure tank cars, which carry most of the commodities that require the special protection. Tank Test Pressure For most tank cars, the tank test pressure is denoted in the specification designation. Test pressures are measured in pounds per square inch (psi) using test methods prescribed in the regulations. Test pressures generally range from 60 psi (except in some older cars) for general service tank cars to 600 psi for the highest test-pressure cars. The Ills can be tested for 60 or 100 psi, although most new cars are built to the 100-psi specification. Most new 105 and 112 cars are built to specifications of 300 psi or higher.